The present invention relates to a method for controlling the braking pressure variation in the rear-wheel brakes of a vehicle brake system having electronic control of the brake force distribution (EBD function).
It is prior disclosed to adjust the distribution of brake force to the front and rear axles, which is subjected to dynamic influences, by means of electronically controlled hydraulic valves. Principally, the rotational behavior of the wheels is measured by wheel sensors and evaluated to limit or adjust the brake slip of the rear wheels. As is known, an excessively great amount of brake slip on the rear wheels could jeopardize driving stability of the vehicle and cause skidding of the vehicle.
EP 0 563 740 A 1 discloses a method of controlling brake force distribution wherein the brake force variation on the rear-wheel brakes shall be controlled appropriately in cooperation with an ABS system in order to transmit the maximum possible brake force to the rear wheels, wherein the braking pressure is transmitted from the brake master cylinder to the rear wheels without changes until a braking pressure limit value and, starting from the braking pressure limit value, the braking pressure to the rear wheels is load-responsively reduced by means of a brake force control valve. The degree of braking is sensed during a braking operation, and the function of the brake force control valve will be eliminated by way of an electromagnetic valve when the braking degree becomes lower than a preset braking degree limit value and the brake force control valve will start to operate when the braking degree is higher than the braking degree limit value, and the braking pressure limit value will be corrected to adopt a lower value when the ABS system for the rear wheels responds.
By means of electrically controllable hydraulic valves which are inserted in the pressure fluid conduit from the master cylinder to the rear-wheel brakes in a brake system of this type, braking pressure and, thus, brake slip is limited to an allowable degree by switching the valves over and, hence, shutting off the pressure fluid conduit to the rear-wheel brakes. This phase where pressure is maintained constant is followed by a new pressure increase phase in many cases, for example, in the event that the initial pressure is increased, or when the brake slip on the rear wheel has decreased again.
The commencement of EBD control (electronic brake force distribution control) frequently occurs in a relatively steady, balanced phase of the braking operation or partial braking operation (in contrast to a panic stop, for example). Noises which are caused by valve actuation in this phase are audible and may irritate the driver and passengers. Such noises reduce the comfort in any case.
German patent application No. 33 01 948 discloses a method of electronically controlling the brake force distribution wherein the brake slip and the longitudinal deceleration of the vehicle are determined from the rotational behavior of the individual wheels, and wherein the brake slip on the rear wheels is limited to a defined percentage of the brake slip on the front wheels by actuation of the braking pressure control valves. German patent application No. 33 06 611 additionally provides a possibility of reducing the braking pressure on the rear axle by way of a hydraulic, electronically operable valve which opens a conduit from the rear-wheel brakes to a pressure supply reservoir.
It is an object of the present invention to provide a method of electronically controlling the brake force distribution which ensures an effective brake force distribution, more particularly, an effective introduction of braking pressure into the rear-wheel brakes without disturbing effects or side effects such as noise development. Compared to conventional systems where disturbing noises occur on valve actuation, one objective was to reduce valve switching noises and thereby increase the comfort.
The method of the present invention include the following steps. During a braking operation, i.e. partial braking operation or ABS control operation, the vehicle deceleration is sensed and evaluated as a control criterion, and that an increase in the braking pressure in the rear-wheel brakes after the commencement of an EBD control function (EBD commencement) is only permitted if (a) the vehicle deceleration after EDB commencement has risen above a predetermined minimum value, and/or if (b) the presence of criteria which indicate brake fading, the imminent risk of fading, or brake malfunction is detected, and/or if (c) the vehicle deceleration exceeds a high limit value which is critical for the safety of the vehicle. A combination of the conditions (a) and (b) or the conditions (a) and (c) has proved to be especially favorable.
The method of the present invention is based on the knowledge that it is sufficient to allow further braking pressure buildup in the rear-wheel brakes only under the condition that defined conditions or criteria are satisfied. In case (a), this means that a higher rear-wheel braking pressure is necessary because, for example, the driver requests a greater deceleration or has initiated increase in braking pressure by application of a higher pedal force. In this case, braking pressure increase on the rear axle can be started when the vehicle deceleration after EBD commencement has risen by a defined deceleration value of e.g. 0.2 g or more. This is an indicator of situation (a).
In an embodiment of the method of the present invention, a vehicle deceleration increase ranging between 0.15 g and 0.3 g, for example a value of roughly 0.2 g, is predetermined as a minimum value, the exceeding of which is evaluated as a criterion for allowing a braking pressure increase in the rear-wheel brakes, where xe2x80x98gxe2x80x99 means the constant of acceleration due to gravity. This is an example for the situation (a).
In another embodiment of the method of the present invention, a criterion or an indication of brake fading, an imminent risk of fading, or a defect is obtained by means of a counter, the content of which counter is varied during a braking operation or during brake actuation according to a predetermined time frame (for example, upon each working clock, a multiple of the working clock, each loop or a multiple). The counter content is increased in the predetermined time frame by respectively a value which depends on the increase in vehicle deceleration. Decrease of the counter content is most simply carried out by an invariable frame. It can also be appropriate to reduce the counter content with declining brake temperature or in dependence on measured quantities which represent the brake temperature, on the vehicle speed, etc.
It has proven suitable to have the increase of the counter content set in with the rise in the vehicle deceleration by roughly 0.1 to 0.15 g per frame step. With higher increases in vehicle deceleration, the counter content is increased more quickly in comparison to the change at a low increase in the vehicle deceleration.
Decrease of the counter content can be linked logically to the release of the brake or uncoupling of the brake pedal which can be seen, for example, in the change in the signal of the brake light switch.
It is also provided within the scope of the present invention to vary the counter rate in dependence on the vehicle speed. Further, it is appropriate to produce a model of the brake temperature in dependence on the brake actuation signals and other influencing variables because the occurrence of brake fading becomes more likely as the temperature increases.
With a particularly high vehicle deceleration of e.g. more than 0.45 g, which can only be caused by a very vigorous brake actuation, stopping or delaying the braking pressure buildup on the rear axle is principally given up in favor of decelerating to achieve an especially short stopping distance for reasons of safety.